In circulating fluid bed boilers, fuel combustion is typically carried out in the presence of a bubbling bed of silica sand, feldspathic sand and/or calcined limestone. Air is forced through this bed of fuel and sand, and/or limestone to fluidize it, and, as the air velocity leaving the primary furnace zone increases, the fuel ash from combustion and other small particles become entrained in this upward flow of the flue gas. The hot ash and other material carried by the flue gas are then removed in a primary collector and are collected in a storage chamber such as a particle storage hopper or other area of spasmodic flow/stagnation, before being recirculated back to the fluid bed.
These particles move through the storage chamber or other receptacle at varying, low flow transport rates while either at a high temperature or subject to high temperatures. This combination of low flow rate and high temperature causes the particles, which are coated with fuel ash, to contact each other and form weak physical bonds or to agglomerate. The formation of these weak bonds or agglomeration is due to the surface of the particles having a low eutectic point or ash softening temperature. This low value is caused by the high alkali content, specifically sodium and potassium compounds, formed during combustion of the boiler fuel. The agglomerated particles, subjected to high temperatures, then begin to sinter or stick together through bond densification thereby forming a strong physical/chemical bond. Such agglomeration or sintering not only occurs between particles thereby creating large clumps of mass, but those particles also stick to the walls of the storage chamber or other receptacle wherever the particles are subjected to spasmodic flow or are stagnant. Consequently, as one can well imagine, this characteristic severely affects the ability to recirculate these particles which oftentimes leads to forced boiler outages.
To counteract this problem, such solutions as lowering the level in the storage chamber, adding limestone in a sand bed media unit to raise the softening point on the particle surface and changing the chemical makeup of the fuel and/or sand/limestone have helped to a degree, but sintering is still a problem. Other solutions such as using sootblowers or sparge pipes have been attempted but once the particles agglomerate (form a weak bond) or sinter (form a strong bond), these solutions did nothing more than simply blow holes in the now-solid material, they did not prevent the situation from occuring in the first place.
It is thus an object of this invention to prevent the agglomeration or sintering of ash-coated particles and particularly when these particles are collected and stored (albeit temporarily) in a storage chamber of a circulating fluid bed boiler or other receptacle where the particles are subjected to spasmodic flow or are stagnant. It is a further object of this invention to prevent such agglomerating or sintering before it occurs rather than to treat the particles after they have already consolidated.